It is common knowledge that wind turbines are protected against lightning strikes, and it is also common knowledge that wind turbine blades are provided with lightning down-conductors arranged either on the outer surface of the blades or inside said blades. The latter type is provided with so-called lightning receptors, which are metallic through-going connections between the inner lightning down-conductor of the blade and the outer surface of said blade. The purpose of these receptors is to “attract” the lightning so that the lightning current can be guided downwards through the lightning down-conductor mounted inside the wind turbine blade. Often, the wind turbine blades with externally mounted down-conductors are not provided with separate lightning receptors as said down-conductors act as receptors, per se.
Previous ways of solving the down-conducting of lightning have not specifically considered that a pitch bearing, if any, can be damaged by a strong lightning current, and typically the current is conducted through the down-conductor of the blade to the blade root and from there through the pitch bearing to the rotor hub. From the rotor hub, the lightning current is conducted into the nacelle/turbine top section to the turbine tower and downwards into the ground.
One disadvantage of conducting the current through the pitch bearing is that bearing rollers etc. can be damaged by a strong lightning current generating electrical arcs on its way down through the bearing with the result that a welding-like effect occurs which can damage the surface of the bearing. Once the surface of a bearing roller etc. has been damaged, the bearing is quickly worn down, which in time results in large repairs involving shut downs of the wind turbine. Even weak currents passing through the pitch bearing can cause small sparks and migration of material between the pitch bearing members moving relative to each other.
The most common strikes of lightning occur when the potential difference between a negatively charged portion of a thundercloud and a positively charged area of the ground beneath the cloud grows sufficiently significant and causes a breakdown of the isolation of the strata of air separating the areas of opposite electrical charge. The phenomenon also arises between a positively charged portion of a thundercloud and a negatively charged area on the ground.
Before the actual lightning strike can occur, a descending “channel” is generated, also called a “leader”, of negatively charged air molecules in the direction towards the ground. Often, the leader propagates gradually downwards in steps of 20 to 100 m and is therefore referred to as a “stepped leader”. When this leader is sufficiently close to the ground, the intensified electrical field between the end of the descending leader and the ground generates one or more upward leaders of positively charged air molecules towards the descending leader. These upward leaders usually extend upward from objects projecting from the ground, for example wind turbines, trees, buildings, flag poles etc. When the two leaders meet, the system short-circuits and the actual main charge of the lightning, also referred to as a “return stroke” occurs. The phenomenon can likewise occur in reverse order, where the leader, also referred to as the “stepped leader”, propagates from the ground or particularly high objects on the ground and moves towards the charged area of the thundercloud.
DE 4436197 A1 discloses a solution where a lightning current is diverted from the pitch bearing and into the nacelle through a lightning conductor member defining spark gaps with the lightning down-conductor of the blade and an electrically conducting ring on the nacelle, respectively. Electric arcs are generated by the lightning strikes in these spark gaps with the effect that the lightning current can be conducted downwards into the ground. Thus, electrical connections are only generated between the lightning down-conductor of the blade and the ground during the main charge of the actual lightning strike where electrical arcs are generated in the spark gaps. As mentioned previously, leaders are generated prior to the actual main discharge, and the electrical isolation between the blade and the nacelle implies that there is a risk of the leaders passing through other uncontrollable paths between the individual structural members—for instance from the nacelle through the main shaft bearing or the rotor bearings to the rotor hub and from there through the pitch bearing of the blade to the lightning down-conducting arrangement of the blade. Subsequent to the formation of electrical leaders passing through possible uncontrollable paths in for example the structural members, the main discharge of the lightning or parts thereof follows the path of the leader generated through the structure to the ground. Such a main discharge or parts thereof may cause minor or major damages on the structure or parts thereof when it is not guided through a lightning down-conductor in a controllable way.
WO 01/86144 A1 discloses a wind turbine with a spark gap between a lightning down-conductor in the blade and the nacelle, and for where an electrical connection exists between the lightning down-conductor and the rotor hub for a continuous electrostatic discharge of the blade. In this structure, the pitch bearing of the blade is not protected against being passed by a discharge current.